By Brendan Conway

We’ll have to wait til next week for the final tally, but barring some huge move in Friday’s fund flows, preliminary BlackRock (BLK) data show August was the biggest investor outflow from exchange-traded funds in more than three years.

The prime culprit is the ETF market’s biggest and most widely traded fund, the $137 billion SPDR S&P 500 (SPY). The ETF had shed a whopping $13 billion alone, through Thursday. Big outflows from stock & bond ETFs overshadowed inflows from international equity ETFs, which took in $5.6 billion.

Questions about when and if the Federal Reserve will reduce its stimulus efforts have dominated the past month, with the threat of U.S. military strikes against Syria fraying nerves earlier this week. The S&P 500 has closed Thursday down 4.2% since hitting its all-time high on Aug. 2, and was on pace for its worst monthly decline since May 2012.

Near the top of the bond-outflow heap lately: iShares 3-7 Year Treasury Bond ETF (IEI), iShares iBoxx $ Investment Grade Corporate Bond ETF (LQD) and iShares iBoxx $ High Yield Corporate Bond ETF (HYG), which XTF data show have shed anywhere from $790 million to $2.4 billion in the last month.

About Focus on Funds

As exchange-traded funds and other investing vehicles have ballooned in number, the task of figuring out what works well and what doesn’t has only gotten harder. Barrons.com’s Focus on Funds looks under the hood of ETFs, mutual funds and hedge funds for overlooked values, actionable ideas and the latest pitfalls for fund investors.